In recent years, there has been widespread introduction of systems in which tags storing article information are attached to articles, including cattle and other living beings, and in which this information is read by non-contact means to manage data. An example of such a system is called an RF (radio frequency) ID system. In such a system, wireless signals of power approximately 1 W in the UHF band (860 to 960 MHz) are used to transmit signals from a reader/writer.
On the tag side, power to start processing is generated based on this signal, and a response signal is returned to the reader/writer from the tag according to instruction information comprised by the signal. By this means, information within the tag can be read by the reader/writer.
A tag attached to an article comprises an antenna, and a functional chip connected to the antenna. The communication distance between the tag and the reader/writer depends on the gain of the antenna incorporated within the tag, the chip operating voltage, and the surrounding environment, but is approximately 3 meters.
The interface portion of the LSI chip 20 mounted in a tag can be represented as equivalent to a parallel connection of a resistance Rc (for example, resistance value=1000Ω) and a capacitance C (for example, capacitance value=0.7 pF), as shown in FIG. 1 explaining matching of the antenna and the LSI chip. On the other hand, the antenna mounted on the tag (called the tag antenna) can be represented as equivalent to a parallel connection of a resistance Ra (for example, resistance value=1000Ω) and an inductance L (for example, inductance value=40 nH).
As a result of parallel connection of the two, the capacitance value C and inductance value L are in resonance, and as indicated by equation (1), the imaginary component is substantially zero and matching is achieved, so that the power received by the tag antenna 10 is adequately supplied to the LSI chip 20.
                              f          0                =                  1                      2            ⁢            π            ⁢                          LC                                                          (        1        )            
The most basic design for the tag antenna 10 is that of a resonant dipole antenna with total length equal to λ/2; in the UHF band, a length of approximately 150 mm is required. However, in consideration of cases in which such tags are affixed to the ears of cattle and managed, for example, there is a demand to form tags with a maximum diameter of approximately 35 mm.
However, it is substantially impossible to form a λ/2 resonant dipole antenna with such a required size.
The use of a loop antenna configured such as the loop antenna 1 connected to a feed terminal 3 shown in FIG. 2 is conceivable. A loop antenna has characteristics such as those explained for example in Antenna Engineering Handbook, published by Ohmsha, Ltd., Mar. 5 1999, pp. 20-22. That is, it is explained that when a common-mode current flows uniformly in a circular loop 1 connected to a feed terminal 3, as the loop radius is increased, the number of lobes of the power pattern increases.